Caption: Biological clock. Image of the silhouette of a man holding a clock to his chest representing biorhythms. The man is against a glowing orange background. The biological clock (better known as the body clock) is the body's internal timing system. It allows us to live in harmony with the rhythms of nature such as day and night and the seasons. Circadian rhythms, a type of biorhythm, are those processes which happen over a 24 hour period such as sleep and wakefulness. The body clock is not perfect however, and is reset regularly by natural events such as sunrise.